Make the cake and chocolate mousse

1. Preheat oven to 350°F (176°C) and prepare three 8 inch cake pans with non-stick baking spray and parchment paper in the bottom.
2. Add the flour, sugar, cocoa, baking soda, baking powder and salt to a large mixer bowl and combine. Set aside.
3. Add the milk, vegetable oil, vanilla extract and eggs to a medium sized bowl and combine.
4. Add the wet ingredients to the dry ingredients and beat until well combined. Slowly add the hot water to the batter and mix on low speed until well combined. Scrape down the sides of the bowl as needed to make sure everything is well combined.
5. Divide the batter evenly between the prepared cake pans and bake for 22-25 minutes, or until a toothpick comes out with a few moist crumbs.
6. Remove cakes from oven and allow to cool for 2-3 minutes, then remove to a cooling rack to finish cooling.
7. When the cakes have cooled, make the chocolate mousse. Combine the egg yolks, sugar and 1/2 cup (120ml) of heavy whipping cream in the top of a double boiler (or in a metal mixing bowl set over a pot of simmering water). Do not let the water boil, or it’ll get too hot. Occasionally lift the bowl to release steam. Cook the egg mixture, whisking constantly. It’ll be ready when it has thickened, lightened a bit in color and has more volume. It should take about 7-10 minutes and reach 160°F (71°C). When done, remove it from the heat and set aside.
8. Place the chocolate chips in a medium bowl and microwave in intervals of 10-15 seconds, stirring well between each, until melted and smooth. You could also use the double boiler again.
9. Add the chocolate to the egg mixture and whisk until smooth. Set the mixture aside to cool to room temperature.
10. In a mixer bowl, whip the remaining 1 1/4 cups (300ml) of heavy whipping cream and powdered sugar until stiff peaks form.
11. Fold about 1/4 of the whipped cream into the chocolate mixture until combined, then fold about 1/4 of the chocolate mixture into the whipped cream. Slowly fold the remaining chocolate mixture into the whipped cream.

Build the cake

1. Level the tops of the chocolate cakes by using a large serrated knife.
2. The cake can be built in an 8×3 inch cake pan (springform would be ideal) or with a clear cake collar. If you are using a cake pan to build the cake, put a cardboard cake circle in the bottom of the pan. If not using a springform pan, you can put some plastic wrap in the bottom of the pan and up the sides and use it to lift the cake out of the pan when it’s done. Line the sides of the cake pan with parchment paper. The parchment paper should stick up above the top edge of the pan, since the cake will probably be a little taller than your pan. Add the first layer of cake into the bottom of the cake pan. If you are using a cake collar, place the bottom cake on a cardboard cake circle or a serving plate and wrap the cake collar around it and secure in place.
3. Add about half of the chocolate mousse and spread into an even layer.
4. Add the second layer of cake on top of the mousse, then add the remaining mousse and spread into an even layer.
5. Top the cake with the final cake layer. Refrigerate the cake until the mousse is firm, 4-5 hours.

Finish it off

1. Make the whipped cream by adding all the ingredients to a large mixer bowl and whipping on high speed until stiff peaks form.
2. Remove the cake from the fridge and either remove it from the pan by removing the springform pan sides, lifting the cake out of the pan with the plastic wrap, or removing the cake collar.
3. Frost the outside of the cake with the chocolate whipped cream and pipe swirls around the outer top edge of the cake.
4. Finish off the cake with some chocolate sprinkles, then refrigerate until ready to serve. Cake is best when well-covered in the fridge for 3-4 days.